Air kerma transmission factors of scattered X-rays in the maze of a linac room for lead shield

Spectra of scattered X-rays in the maze of a Linac (X-ray energies of 4, 6, and 10 MV) room were estimated by means of the Monte Carlo simulation, and air kerma transmission factors of the X-rays scattered through a lead shield were evaluated based on those spectra. Spectra of scattered X-rays showed a maximum in the energy area below 200 keV. The higher the accelerated electron energy, also, the smaller the scattering angle that tended to spread to the higher energy area of the distribution of spectra. The air kerma transmission factor of 120 deg scattered X-rays of 4 MV X-rays obtained in this study was larger than the transmission factors of 124 deg scattered photons of 60Co gamma rays through a lead shield given in ICRP. The air kerma transmission factors of 120 deg scattered X-rays of 6 MV X-rays were smaller than the transmission factors of 90 deg scattered photons of 60Co gamma rays. The air kerma transmission factors of 120 deg scattered X-rays of 10 MV X-rays was slightly larger than transmission factors of 90 deg scattered photons of 60Co gamma rays. Therefore, in the case of a 4 MV X-ray Linac room, the calculation method given in the ''Manual of Practical Shield Calculation of Radiation Facilities (2000)'' causes underestimation of leakage doses. (authors)